a) Field of the invention:
The present invention relates to a semiconductor photo-electric converter, and more particularly it concerns a static induction type semiconductor photoelectric converter.
b) Description of the prior art:
As known semiconductor photo-electric converters, there are photo-conductive light detectors, photodiodes, photo-transistors and like devices. A known bipolar type photo-transistor has a structure similar to that of a known bipolar transistor, and is operative so that those carriers which are generated by light are accumulated in the base region of the transistor to control the emitter current.
On the other hand, a static induction transistor has been proposed by Jun-ichi NISHIZAWA, present inventor, in, for example, U.S. Ser. No. 817,052 and by Jun-ichi NISHIZAWA et al in U.S. Ser. No. 576,541, which is capable of exhibiting a non-saturating type current versus voltage characteristic in contrast to known bipolar transistors and known field effect transistors which exhibit saturating type current versus voltage characteristic. The static induction transistor (hereinafter to be called simply SIT) has greatly advanced the art in the past.
A static induction transistor is designed so that its series resistance from the source region to the intrinsic gate (the region in which the drain current is substantially controlled) is very small, and that it has a large transconductance. The impurity concentration of the channel region is selected low and the width of the channel region is selected small so that the channel region can be pinched off by the depletion layer extending from the gate, so as to form an intrinsic gate of potential harrier type. Also, the gate length is selected short to provide very small series resistance.
Also, in case a potential barrier is produced in the intrinsic gate region, the height of this potential barrier can be controlled by either one of the gate potential and the drain potential, and these constitute a cause for being able to exhibit a non-saturating type current versus voltage characteristic. The SIT has a further convenience that its gate capacitance can be made very small due to a very small gate length structure, so that it is suitable for high-speed and low power dissipation operation. However, no optical static induction type semiconductor device has been proposed yet.
An image pick-up device can be constituted by integrating photoelectric semiconductor elements. Such image pick-up devices vary in kinds. An all-solid image pick-up device requires no application of high voltage or no provision of vacuum. Accordingly image pick-ups which employ a semiconductor charge transfer (charge coupled) device have been proposed. A charge transfer device has the advantages that it has a simple structure and that it has a high packing density. However, the electric charge which forms an output signal is comprised of those carriers which are ionized by exposure to light and which are stored in such region as MOS capacitor. Therefore, if the amount of light irradiated on the device is not abundant, the electric charge which is stored will be small. Therefore, it is typically no possible to obtain a large signal with such devices if the light is weak. Furthermore, leakage the stored electric charge often occurs when it is being transferred.
Another image pick-up device using a MOS transistor as a light-detector has also been proposed. The detection of light, however, is carried out by utilizing the pn junction at the source region as a photo-diode. Therefore, this known image pick-up device has similar problems as described with respect to the charge transfer device, for example, sensitivity to light.